Closing In on the Next Earth

A group of astronomers today announced the discovery of the least massive planet yet detected outside of our solar system. It is lightweight enough--between two and three times the mass of Earth--to almost certainly be rocky like Earth rather than a huge ball of gas. Although the planet orbits too close to its star to be habitable, a new analysis of one of its neighbors suggests a world with deep oceans. All this raises the prospect of discovering an Earth-size planet orbiting at just the right distance from its star to give life a chance. "It's just a matter of time now," says exoplanet specialist Sara Seager of the Massachusetts Institute of Technology in Cambridge.

Planet hunters are an ambitious lot. They're after an Earth-sized, rocky planet orbiting at a comfortable, livable distance from its star. For ease of observation, the exoplanet should periodically pass between the star and astronomers on Earth. They've already bagged huge planets orbiting in a star's habitable zone--where any water would be liquid--and huge planets transiting in front of their stars. They've found small planets too: CoRoT-Exo-7b, whose discovery was announced in February, is twice the diameter of Earth but likely eight times its mass--and far from habitable.

What makes the new world so noteworthy is its small mass. At the Joint European and National Astronomy Meeting being held this week in Hatfield, U.K., a 13-member group led by planet hunter Michel Mayor of the Observatory of Geneva in Switzerland announced that Gliese 581 e, located only 20.5 light-years away in the constellation Libra, is so small that it isn't smothered with gas the way Jupiter or Neptune is.

The group found the new record holder, as most exoplanets have been found, by monitoring the spectral color of its star (Gliese 581) for years. Subtle periodic color shifts trace a star's wiggling induced by a planet's periodic gravitational tugging as it orbits. Three other planets are known to orbit this star, with Gliese 581 e innermost in a 3.15-day orbit. Being that close to even this low-mass, relatively cool star makes the new find too hot a rock to be habitable; any water would have boiled away eons ago.

But the group also announced a correction to their calculated orbit for Gliese 581 e's massive planetary neighbor, Gliese 581 d, bringing it in from an 83-day orbit to a 67-day orbit. That puts it in the habitable zone, where it would be warm enough for any water to be liquid but not so hot as to boil it away. Gliese 581 d "could even be covered by a large and deep ocean," team member Stephane Udry of the Observatory said in a statement. "It is the first serious 'water world' candidate."

More exciting finds should be on the way. In its manuscript submitted to Astronomy & Astrophysics, the group shows that its current instrumentation, a high-resolution spectrograph linked to the European Southern Observatory's 3.6-meter telescope at La Silla Paranal Observatory in Chile, could detect a planet twice the mass of Earth all the way out in the middle of the Gliese habitable zone. So, given time, says Seager, habitable Earths will turn up. With some luck, one will be aligned to pass in front of its star, letting astronomers decipher more about the next Earth from the starlight passing by.